1. Field of the Invention
The present invention relates to integrated passively modelocked fiber lasers and methods for constructing integrated passively modelocked fiber lasers. In particular, the present invention relates to a low-cost, high-reliability, passively-modelocked fiber laser constructed without non-fiber internal polarization manipulating elements.
2. Description of the Related Art
Techniques for generating short and ultrashort optical pulses in optical fiber lasers have been known for a number of years and have been used in many applications. In general, short pulses in fiber lasers are produced using one of the following techniques: active mode-locking, as described by Carruthers et al., Optics Letters, Vol. 21, p. 1927 (1996); passive-modelocking using saturable absorbers, as described by Ober et al., Optics Letters, Vol. 18, p. 1532 (1993); passive modelocking relying on Kerr-type nonlinearities, as described by M. Hofer et al., Optics Letters, Vol. 16, p. 502 (1991); or a combination of these three techniques. Passive modelocking using saturable absorbers is the simplest approach and is most amenable to the manufacturing of ultra-compact devices, as required when using short-pulse fiber lasers in OEM (original equipment manufacture)-type mass applications.
In the context of passive modelocking using saturable absorbers, ultra-low-cost, highly reliable fiber lasers with as few as possible optical components are highly desirable. While ultra-low-cost fiber lasers (such as those described by: Zirngibl et al., Electronics Letters, Vol. 27, p. 1734 (1991); Lin et al., U.S. Pat. No. 5,436,925 (1995); Barnett et al., Optics Letters, Vol. 20, p. 471 (1995); Hofer et al., Optical Society of America Conf. on Optical Fiber Communication, OFC 1996 paper TuB3 (1996); Loh et al., IEEE Photonics Technology Letters, Vol. 5, No. 1 (1993); Cunningham et al., U.S. Pat. No. 5,701,327; Minden, U.S. Pat. No. 5,488,620 and Tsuda et al., Conf. on Lasers and Electro-Optics, paper CFD2, p. 494 (1996)) as well as highly reliable fiber lasers (such as those described by DeSouza et al., Electronics Letters, Vol. 29, p. 447 (1993) and Fermann et al., Optics Letters, Vol. 19, p. 43 (1994)) and Fermann et al., U.S. Pat. No. 5,627,848 have indeed been manufactured, fiber laser designs that combine these two features have not been developed to date.
Ultra-low cost fiber lasers can, for example, be constructed by using a saturable absorber in a fiber ring cavity as described in Zirngibl et al. and Lin et al. cited above. These two references use low-birefringence fiber to form the cavity, which leads to inherent stability problems due to polarization drifts in the cavity.
Alternatively, ultra-low cost fiber lasers can be constructed by using a saturable absorber as an end mirror in a Fabry-Perot-type cavity as described in the above cited work by Barnett et al., Hofer et al., Tsuda et al., Loh et al., Cunningham et al. and Minden, and by Sharp et al. in U.S. Pat. No. 5,666,373. The Fabry-Perot cavities are all quite similar as they are also constructed with low-birefringence fiber with the same stability problems as the two ring cavities described above. Only in the work by Minden et al. is the stability problem not an issue since the object there is to generate a pseudo-random pulse train. In contrast, most applications of mode-locked pulses require a very stable pulse train void of any time-dependent (or long-term) changes in the polarization output state that are inherent to the use of low-birefringence intra-cavity fiber. The early work in this area dates to the publication by Loh et al. in 1993, which illustrates the essential elements of an integrated passively mode-locked fiber laser. Fermann et al '848 also suggested low-cost cavity designs, albeit with bulk polarizers or paddle-wheel polarization controllers. In particular Fermann et al '848 suggested the use of cladding-pumped fiber and the use of a partially reflecting saturable absorber for output coupling. The additions by Sharp et al and Cunningham et al specifically only relate to the exact design of the saturable absorber. The addition by Sharp et al relates more specifically to a saturable absorber formed integrally as a low reflector mirror. This possibility was also discussed by Fermann et al '848, albeit not specifically for Tm-doped fiber. Equally, semiconductor processing techniques also allow low reflectivity saturable absorbers mirrors that are not formed (or grown) directly on mirror structures. None of the above references, however, address the fact that the use of low-birefringence fiber leads to an inherent polarization stability problem in such lasers.
Of course the intra-cavity polarization state in Fabry-Perot or ring cavities can be controlled by the use of fiber paddle wheel polarization controllers. However, such polarization controllers are also inherently unstable and should be avoided. Polarization controllers as used in mode-locked lasers can serve a variety of functions. In the presence of a polarization-dependent loss, polarization controllers adjust the intra-cavity loss to a level where mode-locking by a saturable absorber is stable. Further, though the polarization state of the light of a reciprocal Fabry-Perot laser is linear at each end of the cavity, the direction of the polarization direction is undetermined and controlled by intra-cavity polarization controllers.
The most subtle requirement for intra-cavity polarization controllers in the presence of low-birefringence fiber is the fact that the internal round-trip phase-delay between the light propagating along the two eigenstates of polarization inside the fiber cavity needs to be adjusted to be close to 2.pi. to obtain stable mode-locking with a fixed polarization state. Unless the phase delay is close to 2.pi., the light along the two intra-cavity polarization directions can beat inside the cavity producing a highly undesirable, time-varying, inherently noisy polarization output (as reported by Hofer et al., OFC 1996, paper TuB3). For example, if the linear phase delay between the two polarization eigenmodes in the cavity is 2.pi., the output polarization switches between two orthogonal polarization states in each round trip, as it takes two round trips to reproduce the original polarization state in the cavity. The polarization beating problem is particularly significant in the femtosecond regime, as the pulses are very short and pulses propagating along the two eigenmodes of the cavity separate from each other very rapidly. In the picosecond regime the pulses stay together much longer and should therefore be less sensitive to polarization beating.
While the linear phase delay between the polarization eigenmodes can be adjusted to be close to 2.pi. by using polarization controllers (which will tend to lock the eigenmodes in phase), such a control is not easily incorporated in a pre-determinate fashion (as required for the mass-production of such devices). Moreover, any temperature and pressure variation will lead to a change in the linear phase delay and the direction of the output polarization; thus the whole system also requires continuous and sophisticated stabilization schemes.
On the other hand highly reliable fiber lasers have been constructed by resorting to highly birefringent fiber as reported by Fermann et al. (1994), DeSouza et al., Electron. Lett., vol., 29, p. 447, 1993, and Fermann et al. U.S. Pat. No. 5,627,848. In these particular systems a single polarization mode is selected by employing bulk polarizing elements as disclosed by Fermann et al., such as bulk polarizing elements in conjunction with bulk polarization controllers, or simply bulk polarizing elements as disclosed by DeSouza et al. However, whereas no continuously controllable polarization stabilization schemes are required in these cavity designs, the use of bulk-optic polarization-manipulating elements greatly increases the complexity of these systems and their manufacturing cost.
Therefore, to minimize the manufacturing cost of mode-locked fiber lasers, bulk polarization-controlling elements or paddle-wheel fiber-type polarization controllers need to be eliminated from the cavity.